The fundus of the eye, or retina, is a complex layered structure arranged in an approximately spherical shape at the back of the eyeball. It contains the light sensing rods and cones that enable vision. It is nourished by oxygenated blood supplied through arterioles and removed through venules. The nerve impulses from the rods and cones are directed to the brain through the optic nerve on the fundus, corresponding to the blind spot.
Direct visual observation of the retinal fundus can be accomplished using an ophthalmoscope, an instrument that has been around in various forms for over 150 years. The ophthalmoscope employs a light source, means for coupling the light into the eye through the pupil, and means for collecting light reflected back from the fundus and presenting an image of the fundus to the observer. The eye responds to continuous light by constricting the pupil size and so reducing the amount of light available to form an image. For this reason, the eye pupil may have to be chemically dilated using a mydriatic.
A fundus camera is similar to the ophthalmoscope but provides a permanent record of the fundus image in the form of a photograph. It also enables the use of a short, powerful flash of light to replace the continuous light required for the ophthalmoscope, and so sometimes avoiding the need for a mydriatic. The fundus camera uses an electronic image sensor such as a charge-coupled device (CCD) or a scientific grade CMOS image sensor, and the image is stored electronically. It may be displayed on a monitor or printed out as a photograph.
The fundus image is dominated by the appearance of the optic nerve and the vascular structure of arterioles and venules. It is substantially of the color red, this coming from the blood, with some regions having an orange or yellow bias. The ophthalmologist is able to use this visual image to aid in the diagnosis of the health of the eye. Thorough diagnosis requires the use of a battery of other oculometric instruments in addition to the fundus camera.